The solar transmission (T.sub.s) of typical soda-lime glass is about 85%. For certain applications, for example, a solar heliostat glass for backside reflecting mirrors, glasses demonstrating greater solar transmissions would be highly desirable. Moreover, it has been recognized that some glass formulations are prone to develop discoloration ("browning") therein after exposure to radiation, this coloration reducing the overall solar transmission of the glass. An interaction occurring between the glass and the radiation can cause electronic changes in the glass. If the source of the radiation is sunlight, the changes are commonly referred to as resulting from solarization. Solarization reactions within a glass appear to be caused by a transfer of electrons taking place between ions capable of donating and accepting charges. Damage to the glass network may also occur when the glass is exposed to very high energy radiation. However, such damage is beyond the scope of solarization. In any event, glasses subject to substantial solarization are self-evidently not useful for solar applications where high transmissions are demanded.
The spectral transmission of soda-lime glass over the visible region (.about.400-780 nm) is essentially transparent for most applications and certainly "clear" to the eye unless long light path lengths are used such as, for example, viewing a large article "edge on". The coloration of glass, when observed, is normally the result of contamination arising from the presence of tramp iron oxides in the composition. These iron oxides customarily have their source as contaminants in such batch ingredients as sand, feldspars, limestone, etc., or they may be inadvertently picked up during batch and cullet handling.
The iron oxide conventionally contains ferrous (Fe.sup.+2) and ferric (Fe.sup.+3) ions with characteristic absorptions in the near infrared region of the radiation spectrum, i.e., about 1100 nm, and in the ultraviolet region, i.e., about 380 nm. The presence of a high percentage of Fe.sup.+3 ions gives rise to a yellow to yellow-green coloration, whereas the presence of substantial amounts of Fe.sup.+2 ions yields a blue coloration and causes the glass to absorb strongly in the infrared region. A typical soda-lime glass contains about 0.1% iron, expressed as Fe.sub.2 O.sub.3, with about 30% of the iron content being present in the Fe.sup.+2 state. The desire to transmit solar energy is not limited to the visible portion of the spectrum. Rather, the need is for high transmission over the spectral region of 350-2100 nm. Consequently, the strong absorption by Fe.sup.+2 ions in the near infrared portion of the spectrum is undesirable.
Numerous drawing processes are known to the art wherein glass sheet is formed directly from a melt and, in most of those processes, the surfaces of the glass sheet are not contacted by molds or rollers until after the glass has cooled sufficiently to resist surface marking. Three of the most widely known of those processes are the Colburn process, the Fourcault process, and the Pittsburgh Plate or Pennvernon process. Each operation utilizes rollers to draw sheet up from a glass melt but can provide glass of near-optical quality and without surface markings. A recently-developed downdraw process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609 which is especially suitable for forming glass sheet of controlled uniform thickness and optical quality.
Unfortunately, however, each of those sheet drawing processes requires holding large volumes of glass at relatively low temperatures to secure the necessary sheet forming viscosities, viz., about 10.sup.4 -10.sup.6 poises. Moreover, those volumes of molten glass are required to be in extended contact with the refractory metals or ceramics utilized as the means for forming the drawn sheet. Accordingly, then, those processes impose severe constraints on operable glass compositions because of the formidable liquidus and glass stability problems associated with the handling and processing of glass at relatively low temperatures and high viscosities.
Because of its inherent capability to produce glass sheet of optical quality, the aforementioned recently-developed downdraw process is especially useful to produce glass sheet destined for solar applications or other utility where high transmission is the goal. That process requires a glass exhibiting a viscosity at its liquidus temperature of at least 10.sup.4 poises and preferably about 10.sup.5 poises, and demonstrating long term stability against devitrification and interfacial crystallization in contact with platinum and such refractory ceramics as mullite, sillimanite, zircon, and high density alumina-containing refractories customarily employed to contain or form the molten glass. The growth of a crystalline layer at the glass-refractory metal or ceramic interface not exceeding 10 microns in thickness over a contact period of 30 days when the glass melt is at a viscosity between about 10.sup.4 -10.sup.6 poises is deemed to be good long term stability.
Furthermore, inasmuch as a discoloration arising from solarization can be deleterious to the transmission of the glass, additives known to inhibit the action of that phenomenon will be advantageously included in the glass compositions.
Finally, because the primary application for the glasses involves exposure thereof to solar radiation in the ambient environment, good chemical durability and, in particular, high resistance to weathering is demanded.